5.3, Design criteria

With the identification of operational and maintenance functions and the accomplishment of requirements allocation, it is possible to generate detail design criteria. Such criteria constitute specific requirements in the areas of equipment packaging/modularization, standardization, interchangeability, mounting provisions, degree of self-test features and the placement of test points, extent of automation versus manual provisions, repair ver­sus discard levels, safety features, labeling, and so on. These criteria may be stated qualitatively or quantitatively, and are employed as guidelines for the design engineer. Qualitative criteria must support the quantitative goals developed through allocation.

148

Functional Analysis and Requirements Allocation Chap. 5

.> jh

with a given skill is able to perform are defined based on past experience. The specifi­cation of skill level and MMH/OH requirements dictates that equipment design shall be constrained to the extent that anticipated maintenance tasks can be adequately and ef­fectively accomplished within the prescribed limits. This infers that equipment design should consider the incorporation of simple readout devices, the proper layout of equip­ment front panels, automation of complex operating functions, standardization of com­ponents, labeling, and other provisions that will facilitate the ease and simplicity in the accomplishment of maintenance functions. The proper consideration of these provisions is facilitated through the application of good human engineering principles.

The established design criteria must be consistent with system operational require­ments, the maintenance concept, and the factors defined through allocation. Such criteria provide initial guidelines to the design engineer. Through the early phases of system development, design progress is monitored in terms of compliance with these guidelines. This monitoring process is accomplished through day-to-day design liaison activity, re­liability and maintainability predictions, logistic support analyses, and periodic design reviews.

4. An allocated cost target (i.e., design to a cost) will influence reliability, main­tainability, supportability, and other characteristics of design. It may appear to be too costly (from the standpoint of design labor, production material, etc.) to incorporate certain provisions in the design; however, by not doing so, the subsequent operational and maintenance costs may be high. The allocation of criteria for design should be assessed on a continuing basis in terms of total life-cycle cost, and not just one element of cost.

Questions and problems

1. What is meant by functional analysis! Why is it important? What purpose does it serve?

2. Can a functional analysis be accomplished on any system or equipment item?

3. Select a system of your choice and construct a functional block diagram showing three levels of operational functions. Show two levels of maintenance functions. How do operational functions and maintenance functions relate? (Provide an illustration.)

4. Describe the process (or identify the steps) leading from the functional analysis to the iden­ tification of a particular logistics resource requirement.

5. What is the purpose of the block numbering practice in the development of functional flow diagrams?

6. In equipment design, what are the benefits of functional packaging?

7. What is the purpose of allocation? How does it have an impact on system design? To what depth should allocation be accomplished?

8. Briefly describe the steps in reliability allocation; maintainability allocation; the allocation of logistic support factors; and the allocation of cost factors.

9. What is the relationship between reliability allocation and maintainability allocation? (Provide

a few examples,)

Sec. 5.1 System Functional Analysis

125

I

2. That all elements of the system (e.g., prime equipment, test and support equip­ ment, facilities, personnel, data, software, etc.) are fully recognized and defined.

3. That a means of relating equipment packaging concepts and support requirements to given functions is provided. This identifies the relationship between the "need" and the "resources required" to support that need.

4. That the proper sequences and design relationships are established, along with critical design interfaces.

Functional analysis is a logical and systematic approach to system design and de­velopment. It constitutes the process of translating system operational and support re­quirements into specific qualitative and quantitative design requirements. This process is iterative, and is accomplished through the development of functional flow block dia­grams.

Functional flow block diagrams are developed for the primary purpose of struc­turing system requirements into functional terms. They are developed to indicate basic system organization, and to identify functional interfaces. Functional blocks are con­cerned with what is to be accomplished, versus the realization of how something should be done. It is relatively easy to evolve prematurely into equipment design data without having first established functional requirements. The decision concerning which func­tions should be performed by a piece of equipment, or by an element of software, or by a human being, or by a combination of each should not be made until the complete scope of functional requirements has been clearly defined. In other words, not one piece of equipment should be defined or acquired without first justifying its need through the functional requirements definition process.

The functional analysis (and the generation of functional flow diagrams) is in­tended to facilitate the design, development, and system definition process in a complete and logical manner. The functional analysis is based on the definition of system opera­tional requirements and the system maintenance concept, and is subsequently used as the basis for detail design. There are a number of interrelated detail design tools which must "track" the top-level functional analysis (e.g., operational and maintenance func­tional block diagrams). One of the ultimate objectives is to (1) identify system/subsystem functions; (2) identify the method for accomplishing the various functions—manually, automatically, or a combination thereof; and (3) identify the resources required to ac­complish the function. Both the operational and maintenance support aspects, as related to anticipated system life-cycle use in the consumer environment, must be addressed.

A. Functional Flow Diagrams

The translation of system operational and maintenance concepts into specific qualitative and quantitative design requirements commences with the identification of the major functions that the system is to perform, followed by the development of functional flow diagrams. Functional flow diagrams are employed as a mechanism for portraying system design requirements in a pictorial manner, illustrating series-parallel relationships, the

Sec. 5.1 System Functional Analysis

127

Need

System Requirements

t.

2.0

\

Function A

Function B

V

Function D

Top Level (Functions)

3.0

Function C

5.0

Function Ј

First Level (Subfunctions)

4.1

4.2

4.6

4.3.1

4.3.4

\

Second Level

4.3.2

/"4.3.3X

i

Figure 5-1. System functional indenture levels.

Sec. 5.1 System Functional Analysis B. Operational Functions

129

The functional description represents an overall portrayal of the functions that are nec­essary to describe the total spectrum of system activities. Referring to Figure 5-2, the top row of blocks represents the broad spectrum of activity from the definition of system requirements through the operational use and sustaining support of that system in the user's environment. Through the accomplishment of functional analysis, all (or any one) of the blocks can be broken down to the level necessary to identify specific resource requirements.

In Figure 5-2, the gross operational activities in block 9.0 are based on the mission requirements described in Chapter 3. This may constitute a description of the various modes of system operation and utilization. For instance, typical operating functions might include (1) "prepare aircraft for flight," (2) "fly aircraft from point A to point S,^>7 and (3) "recycle aircraft for the next flight." Assuming that block 9.6 represents the second function ("fly aircraft . , /'), applicable subiunctions might apply to the need for com­munications ("accomplish communication for 7 days per week, 6 hours per day, throughout area ABC"), the need for a radar capability ("search for target"), and so on. The functions are stated in action-oriented terms describing the what (versus the how) requirements.

At some point in time as one progresses toward a greater degree of definition, a response to the "hows" is appropriate! How is each of the specified functions to be accomplished, and what resources are necessary? Trade-off studies are initiated, alter­native approaches are evaluated, and a recommendation is proposed. Such proposed resource needs may be stated in terms of equipment, software, facilities, personnel, data, or a combination thereof.

Ultimately, the functional analysis will lead to the identification of specific com­ponents of the system. As a design objective, the grouping and packaging of components by function (i.e., functional packaging) is highly desirable. Figure 5-3 illustrates the process, leading from the identification of functional modes of system operation to the integration of components into three basic physical units: Units A, B, and C of System XYZ. The operational functions incorporated into each of the three units are as indicated in Figure 5-3, Each major function, in turn, is then analyzed and expanded through the identification of subfunctions as illustrated in Figure 5-4. Developing the design concept farther, an analysis of the functions contained within each unit will lead to the identifi­cation of major assemblies.¹

Given a broad functional packaging scheme, it is then possible to allocate perfor­mance parameters, effectiveness figures of merit (FQMs), and related factors from the system level down to the unit level, and possibly to the assembly level- This constitutes a top-down assignment of system-level requirements to the various components of the system, and serves as a basis for subsequent detail design.

^llt should be noted that equipment design is by no means formulated at this time, A gross-level con­figuration is developed for the purpose of allocating requirements. This configuration serves as a starting point and may be verified or may change as a result of subsequent analyses.

4.0-Self-Test Function

5,0-Range information

6.0 -Trans pond ing Function

"XG-Control Function

System

Operational

Requirement '. Activity

Operational

Mode 2

Operational Mode 3

Operational Mode 1

1.0-Bearing Information

2.0-Tone Information

3.0-Control Function

4.0-Self-Test Function

1,0-Bearmg Information

2.0-Tone Information

5,0-Range Information

Function

3.0-Control Function

runcuons

Operational Functions

Suhf unctions

1.1-Bearing

Alarm

2-Azimuth Signals

2.1-Tone Identity

2.2-Tone Volume

5.1-Range Signals

5.2-Range Validity

4.1-Continuous

Self-Test

4,2-Go/No-Go

Indicators

6,1-Receive ln(er-!^Julses

6.2-Transmit Reply

3 A -Power

Transfer

3.2-Channel Selection

1,3-Bearing Validity

5.3-Range

Alarm

5,4-Range Measurements

4.3-lnterruptive Self-Test

Operation

Functional Packaging

Unit A

Unit B

Unit C

Figure 5-3. System XYZ operational functional-flow diagram.

.0

1 0

3,0

4.0

.0

7.0